The present invention relates to a novel, flexible polypropylene resin suitable as thermoplastic elastomer material, and a propylene based elastomer composition containing the polypropylene resin. The present invention also relates to a process for producing an olefin polymer, which may be suitably used for producing the above flexible polypropylene resin and the propylene based elastomer.
Thermoplastic elastomers have been widely used, as energy saving or resource saving type elastomers, for automotive parts, industrial machine parts, electric or electronic parts, construction materials, especially, as replacement for vulcanized rubbers.
In general, olefin based thermoplastic elastomers (TPO) have been produced by a process comprising blending polypropylene and an ethylene-propylene-diene rubber (EPDM) in the presence of peroxide. Such process is described in, for example, Japanese Patent Application Laid-open Gazette (Kokai) N. 61-217747. However, this process is disadvantageous since it requires complicated procedures and is a costly process.
On the other hand, several attempts have been made to lower the production cost, by directly polymerizing a high molecular weight polymer having dynamic properties similar to those of the above TPO. For example, Kokai 49-53983 and Japanese Patent Application Publication Gazette (Kokoku) No. 62-19444 propose a propylene-hexene copolymer, and Kokai 61-179247 proposes an elastomeric polypropylene. However, these polymers are inferior in low temperature properties.
Further, as a method of improving polypropylene in low temperature properties, a two-step propylene/ethylene-propylene polymerization is well known as described in, for example, Kokai 57-50804. However, it is difficult to produce vulcanized rubber like polymer having flexibility and tensile strength sufficient enough for practical use.
On the other hand, polypropylene has been produced using a Ziegler type catalyst. In this case, mainly crystalline isotactic polypropylene is produced with about 10 to 15% of atactic polypropylene as by-product. The atactic polypropylene has a low number average molecular weight (Mn) about 10,000, and thus are not suitable for practical use.
In the meanwhile, the present inventors found that atactic polypropylene having high molecular weight can be readily produced by polymerizing propylene using a catalyst system comprising combination of a solid catalyst component containing, as essential components, magnesium, titanium, a halogen atom and an electron donor; an organoaluminum compound; and an alkoxy group-containing aromatic compound (Kokai 63-243106). The atactic polypropylene is characterized by being soluble to boiling heptane; having a high molecular weight of, generally, 25,000 to 100,000; and having a relatively narrow molecular weight distribution. The atactic polypropylene has good melting properties as rubber like elastomer. However, the atactic polypropylene has poor mechanical strength, resulting in, when used alone, restriction in application for molded materials.
In view of the above situations, it is an object of the present invention to provide a flexible polypropylene resin having excellent dynamic properties as thermoplastic elastomer and having good cost performance, which can be used for, e.g., automotive parts, industrial machine parts, electric or electronic parts and construction materials.
It is another object of the present invention to provide a propylene based elastomer composition having, even without being vulcanized, tensile strength sufficient enough for practical use; having sufficient flexibility and low temperature properties; having low surface tackiness; and having good cost performance.
It is further object of the present invention to provide a process for producing an olefin polymer, which can be used for producing the above flexible polypropylene resin and the propylene based elastomer composition.
The present inventors made intensive studies to achieve the above objects, as a result of the studies, it was found that the above objects can be achieved by a flexible polypropylene containing, at a specific ratio, atactic polypropylene having specific molecular weight and specific molecular weight distribution and crystalline isotactic polypropylene having specific melt-index; and a flexible polypropylene containing, at a specific ratio, atactic polypropylene component having specific intrinsic viscosity and isotactic polypropylene having specific intrinsic viscosity.
Further, the present inventors found that an elastomer composition having physical properties similar to those of the TPO (partially cross-linked) can be obtained by the use of the above-mentioned specific flexible polypropylene homopolymers, even when they are not cross-linked.
Furthermore, the present inventors found that a vulcanized rubber like olefin polymer having physical properties similar to those of the TPO (partially cross-linked) can be obtained by the use of a specific catalyst system to control crystallinity of the resultant polymers, even when they are not cross-linked.
The present invention is based on the above findings.